Toyota Repair: 2000 LE Ssienna Tire Pressure System, wheel speeds, pressure indicator

Question
I have had problems with my tire pressure system on my van.  I had to have one tire replaced and ever since then my low tire pressure indicator comes on when I reach about 70mph or driving for long periods.  I have attempted to reset the system and think I was successful but the indicator continues to illuminate.  I brought my van into the dealership and they said that my tire pressures needed to be adjusted.  How far below the 32-35psi will the system indicate low tire pressure?  What can I do to remedy this problem?

Thank you for your time and expertise.

Answer
The tire pressure may not be the problem in this case, the problem may be that one tire was replaced, especially if the other tires are somewhat worn.
Ideally, all tires are of the same size and brand and if all the pressures are the same they will rotate at the same speed, if one tire is slightly larger it can present a problem because this tire is going to rotate just a little bit slower, the computer compares each tire revolution and if you lose pressure in one tire it will rotate at a different speed than the rest and it will turn the warning light on, so the computer doesn't actually "see" tire pressures but it can detect wheel speeds which are totally dependent on the circumference of the tire which changes with pressure.
The problem may be that the replacement tire,although it is the same size, may deviate just enough to trigger the warning light.
My advice, replace all the tires with the same size and brand.